Intelligent Paging-Channel Scanning Based on Location Relative to Zone of Last Registration

ABSTRACT

Exemplary methods and systems are disclosed herein that may, among other benefits, help a mobile station to conserve power by intelligently determining how often to scan for incoming pages. An exemplary method may be carried out by a mobile station that has most-recently registered with a radio access network in a zone of last registration, and involves the mobile station (a) determining a location of the mobile station in relation to the zone of last registration, (b) using the location of the mobile station in relation to the zone of last registration as a basis for determining a scan period to wait between scans of the paging channel; and (c) periodically scanning the paging channel according to the determined scan period. Furthermore, an exemplary method may be carried out in order to conserve battery power when a mobile station determines that its battery power is low.

BACKGROUND

In a typical cellular wireless communication system, an area is dividedgeographically into a number of cells and cell sectors, each defined bya radio frequency (RF) radiation pattern from a respective base stationantenna. The base station antennae in the cells may then be coupled witha base station controller, which may then be coupled with a switch orgateway that provides connectivity with a transport network such as thepublic switched telephone network (PSTN) or the Internet. When a mobilestation, such as a cellular telephone, pager, or wirelessly-equippedcomputer, is positioned in a cell, the mobile station communicates viaan RF air interface with the base station antennae of a cell.Consequently, a communication path can be established between the mobilestation and the transport network, via the air interface, the basestation, the base station controller, and the switch or gateway.

Further, in some wireless communication systems, multiple base stationsare connected with a common base station controller, and multiple basestations are connected with a common switch or gateway. Each basestation controller may then manage air interface resources for multiplewireless coverage areas (e.g., multiple cells and sectors), byperforming functions such as assigning air interface traffic channelsfor use by mobile stations in the coverage areas and orchestratinghandoff of calls between coverage areas. In turn, a switch and/orgateway may control one or more base station controllers and generallycontrol wireless communications, by performing functions such asreceiving and processing call requests, instructing base stationcontrollers when to assign traffic channels, paging mobile stations, andmanaging handoff of calls between base station controllers.

In general, air interface communications in each sector (or other suchcoverage area) of a cellular wireless communication system can beencoded or carried in a manner that distinguishes the communications inthat sector from communications in adjacent sectors. For example, in aCode Division Multiple Access (CDMA) system, each sector has arespective pseudo-random noise offset or “PN offset” that is used toencode or modulate air interface communications in the sector distinctlyfrom those in adjacent sectors. Analogously, in other air interfaceprotocols, communications in one sector may be distinguished from thosein other sectors by frequency, time, and/or various other parameters.

Each sector may define an air interface “paging channel” on which theserving base station can page a mobile station. For example, pagingchannel may be used to send access probe acknowledgements and trafficchannel assignment messages to served mobile stations. The pagingchannel also defines timeslots in which the base station can sendvarious messages to particular mobile stations. For instance, if anaccess probe is received from one mobile station seeking to connect to asecond mobile station, the base station may send a page via the pagingchannel to notify the second mobile station of the incoming call.

When a cellular wireless communication system seeks to page a mobilestation (e.g., for an incoming call or for some other reason), a switchin the network may send the page message to numerous base stations inthe switch's coverage area, with the hope that when the base stationsbroadcast the page message, the mobile station will receive the pagemessage in one of the associated sectors, and will respond. Furthermore,given the scarcity of paging channel resources, however, many moderncellular networks are arranged to engage in a more targeted pagingprocess known as “zone based paging.”

With zone based paging, a cellular network is divided into paging zones,each with a respective zone ID, and paging is performed on a zone-basis.To facilitate this, each base station in the system may broadcast as oneof its overhead parameters the zone ID for the zone in which the basestation is located. Mobile stations operating in the network may thenprogrammatically monitor the zone IDs indicated in the overhead messagesand may automatically register with the network when they detect thatthey have moved into a new zone, or for other reasons. With thisprocess, the registration records thereby maintained by switches and/orhome location registers will indicate the paging zone in which eachmobile station last registered.

Using the general paging strategy that is implemented in many wirelessaccess networks, the network may make up to three attempts to page amobile station. In particular, the network may first attempt to transmita page record in the paging zone in which the mobile station isregistered (i.e., the base stations in the paging zone in which themobile station is registered transmit the page record). Then, if thefirst attempt is unsuccessful (i.e., the mobile station does notacknowledge the page record), the network makes a second attempt to pagethe mobile station by again transmitting the page record in the pagingzone in which the mobile station is registered, and possibly in adjacentzones as well. If the second attempt also fails, then the network makesa third attempt to page the mobile station, which typically involves asystem-wide transmission of the page record (i.e., in all paging zonesserved by the switch that serves the zone in which the mobile stationlast registered), although it is possible that a third attempt may be ofa different scope as well.

OVERVIEW

Under the IS-2000 protocol, a time division multiple access (TDMA)scheme is used for the paging channel, which is accordingly divided intopaging-channel slots (which may also be referred to as “timeslots”). Inparticular, the paging slot cycle may include a predetermined number of80 millisecond (ms) slots. In IS-2000 systems, the paging slot cycle maybe set in multiples of 1.28 seconds (or 16 slots), and is commonly setto 5.12 seconds (or 64 slots), according to a system parameter referredto as the slot cycle index (SCI).

Each mobile station will typically be assigned a particular slot in theslot cycle during which the mobile station will wake up and check thepaging channel for a page. A serving system may then schedule pages to agiven mobile station during its assigned slot in the slot cycle.Accordingly, to help conserve battery power, existing mobile stationstypically monitor only their assigned slots on a given paging channel.For example, when the slot cycle is 5.12 seconds, mobile station willscan the paging channel every 5.12 seconds for any pages directed to themobile station.

According to an exemplary embodiment, a mobile station may intelligentlydetermine when it can scan the paging channel less frequently. Inparticular, a mobile station may determine its location relative to itszone of last registration, and adjust the amount of time it waitsbetween paging-channel scans based on how far it is from the zone inwhich it last registered. For instance, if a mobile is still located inthe zone in which it registered, then the mobile station may assume thatup to three attempts will be made to send it a page at its currentlocation, and thus may only wake up to scan in its assigned slot inevery third slot cycle. However, if a mobile station is located in aneighbor zone, then the mobile station may assume that a page will onlybe sent to it on the second and third attempt, and thus may wake up toscan in its assigned slot in every second slot cycle. And if a mobilestation is not located in its zone of last registration or a neighborzone, then the mobile station may assume that a page will only be sentto it on the third attempt, and thus will wake up to scan in itsassigned slot in every slot cycle.

In one aspect, an exemplary method involves a mobile station that isoperating in a radio access network: (a) determining a location of themobile station in relation to a zone of last registration, wherein themobile station last registered with the radio access network in the zoneof last registration; (b) using the location of the mobile station inrelation to the zone of last registration as a basis for determining ascan period to wait between scans of a paging channel for a page fromthe radio access network; and (c) periodically scanning the pagingchannel according to the determined scan period.

Further, when a mobile station's battery power is running low, a mobilestation may switch to a power-save mode in which the mobile stationoperates with reduced functionality in order to conserve its remainingbattery power. Accordingly, an exemplary method may be implemented whena mobile station is operating in a power mode. For example, the methodmay further involve the mobile station making an initial determinationto switch to operating in a power-save mode. Then, in response to thedetermination to switch to operating in the power-save mode, the mobilestation may carry out steps (a)-(b) in order to determining the scanperiod. Further, once the mobile station switches to the power-save modethe mobile station may carry out (c) in order to scan the paging channelaccording to the determined scan period.

An exemplary method may be carried out by a mobile station operating ina radio access network that is configured to: (a) initially page themobile station in the zone of last registration, (b) then ifunsuccessful, page the mobile station in both the zone of lastregistration and a first expanded paging area, and (c) then if againunsuccessful, page the mobile station in the zone of last registration,the first expanded paging area, and a second expanded paging area.Accordingly, the step of determining the location of the mobile stationin relation to the zone of last registration may involve the mobilestation determining whether the mobile station is located in either: (a)the zone of last registration, (b) the first expanded paging area, or(c) the second expanded paging area. Then, if the mobile station islocated in the zone of last registration, the mobile station may scanthe paging channel in the mobile station's assigned slot in every thirdpaging-channel slot cycle. Further, if the mobile station is located inthe first expanded paging area, then mobile station may scan the pagingchannel in the mobile station's assigned slot in every secondpaging-channel slot cycle. Yet further, if the mobile station is locatedin the second expanded paging area, then the mobile station may scan thepaging channel in the mobile station's assigned slot in everypaging-channel slot cycle.

In a further aspect, another exemplary method may involve: (a) a mobilestation determining a battery-power level of the mobile station, whereinthe mobile station has most-recently registered with a radio accessnetwork in a zone of last registration; (b) the mobile stationdetermining whether the determined battery-power level is below athreshold level; (c) if the determined battery-power level is above thethreshold level, then the mobile station selecting a first scanningprocess; (d) if the determined battery-power level is below thethreshold level, then the mobile station selecting a second scanningprocess, wherein the second scanning process varies a period betweenscans of a paging channel based at least in part on a location of themobile station in relation to the zone of last registration; and (e)scanning the paging channel according to the selected scanning process.

And in yet a further aspect, an exemplary mobile-station system mayinclude (i) a non-transitory tangible computer-readable medium; and (ii)program instructions stored on the non-transitory tangiblecomputer-readable medium and executable by at least one processor to:(a) determine a location of the mobile station in relation to a zone oflast registration, wherein the mobile station last registered with theradio access network in the zone of last registration; (b) use thelocation of the mobile station in relation to the zone of lastregistration as a basis to determine a scan period to wait between scansof a paging channel for a page from the radio access network; and (c)cause the mobile station to periodically scan the paging channelaccording to the determined scan period.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the present invention is described hereinwith reference to the drawings, in which:

FIG. 1 shows a simplified block diagram of a wireless communicationsystem in which an exemplary embodiment may be employed;

FIG. 2 is a block diagram illustrating a portion of a coverage area ofradio access network in which an exemplary embodiment may be employed;

FIG. 3 is a flow chart illustrating a method, according to an exemplaryembodiment;

FIG. 4 is a flow chart illustrating another method, according to anexemplary embodiment;

FIG. 5A is a flow chart illustrating a method for registration andscanning while in a power-save mode, according to an exemplaryembodiment;

FIG. 5B is a flow chart illustrating another method for registration andscanning while in a power-save mode, according to an exemplaryembodiment; and

FIG. 6 is a block diagram illustrating movement of an exemplary mobilestation through a coverage area of a radio access network.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention are described herein. Itshould be understood that the word “exemplary” is used herein to mean“serving as an example, instance, or illustration.” Any embodimentdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other embodiments. Further, those skilledin the art will understand that changes and modifications may be made tothese embodiments without departing from the true scope and spirit ofthe invention, which is defined by the claims.

The present invention will be described by way of example with referenceto Code Division Multiple Access (“CDMA”) communications in general, andto IS-2000 communications in particular. As described below, IS-2000applies to both circuit-cellular and packet-data communications. UnderIS-2000, packet-data communications are referred to as “1X-RTT”communications, also abbreviated as just “1X.” However, since IS-2000supports both circuit voice and packet data communications, the term 1X(or 1X-RTT) is sometimes used to more generally refer to the IS-2000 airinterface, without regard to the particular type of communicationcarried. It should be understood that the present invention may apply toother wireless voice and data protocols including, without limitation,IS-95, GSM, IS-856 (EVDO), which, together with IS-2000 are consideredherein, individually or in any combination, to comprise a CDMA family ofprotocols.

According to an exemplary embodiment, a mobile station determines howoften to scan the paging channel based on the mobile station's locationrelative to the zone in which the mobile station last registered withthe radio access network (RAN). When the mobile station is operating ina RAN that implements a zone-based paging scheme and possibly in othersituations as well, the mobile station may intelligently adjust theperiod between paging-channel scans in an effort to conserve batterypower. It should be understood, however, that an exemplary embodimentmay provide other benefits in addition or in the alternative to powersaving, and accordingly may be implemented in other scenarios and forother reasons, without departing from the scope of the invention.

Generally, the RAN is configured to page a mobile station lessfrequently in locations that are further from where the mobile stationlast registered with the RAN. Accordingly, when the mobile station iscloser to its zone of registration, the mobile station will typicallydetermine that it can scan the paging channel less frequently andconserve its battery power, and vice versa. For example, under anexemplary zone-based paging scheme, a page may be sent in the zone inwhich a mobile station last registered on the first, second, and thirdattempt to send a page (if a second and/or a third paging attempt isnecessary). Accordingly, if a mobile station is located in its zone oflast registration, the mobile station may determine that it only needsto wake up once every three paging channel slot cycles in order toreceive a page. On the other hand, if the mobile station is in a zoneother than its zone of last registration, then the mobile station mayscan once every other slot cycle or even once in every slot cycle,depending upon how far the mobile station is from its zone of lastregistration. Other examples are also possible.

I. Exemplary Network Architecture

FIG. 1 shows a simplified block diagram of a wireless communicationsystem 100 in which an exemplary embodiment may be employed. Thecommunication system 100 is configured to provide wireless service to amobile station 102, and includes a base transceiver station (BTS) 104, abase station controller (BSC) 106, and a mobile switching center (MSC)108. Collectively, BTS 104, BSC 106, MSC 108 may be referred to as aRAN. Such a RAN may be operated by a service provider, and may alsoinclude additional BTSs, BSCs, and/or MSCs 108, and possibly otherentities as well. As shown, mobile station 102 communicates over an airinterface 103 with a BTS 104, which is then coupled or integrated with aBSC 106. Transmissions over air interface 103 from BTS 104 to mobilestation 102 represent the forward link to the mobile station, whiletransmissions over interface 103 from mobile station 102 to BTS 104represent the reverse link.

BSC 106 is in turn connected to MSC 108, which acts to controlassignment of air traffic channels (e.g., over air interface 103), andprovides access to wireless circuit-switched services such ascircuit-voice and circuit-data (e.g., modem-based packet data) service.As represented by its connection to a public switched telephone network(PSTN) 112, MSC 108 is also coupled with one or more other MSCs or othertelephony circuit switches in the operator's (or in a differentoperator's) network, thereby supporting user mobility across MSCregions, and local and long-distance landline telephone services. Alsoconnected to MSC 108 is home location register (HLR) 110, which supportsmobility-related aspects of subscriber services, including dynamictracking of subscriber registration location and verification of serviceprivileges.

As shown, BSC 106 is also connected to a packet data serving node (PDSN)116 by way of packet control function (PCF) 114. PDSN 116 in turnprovides connectivity with a packet-switched network 118, such as theInternet and/or a wireless carrier's private core packet-network.Sitting as nodes on network 118 are, by way of example, anauthentication, authorization, and accounting (AAA) server 120, amobile-IP home agent (HA) 122, and a remote computer 124.

With the arrangement described above, a mobile station 102 can engage incellular voice and/or packet-data (e.g., 1X-RTT or EVDO) communications.Taking an originating call from mobile station 102 as an example, mobilestation 102 first sends an origination request over air interface 103and via the BTS 104 and BSC 106 to MSC 108. The MSC then signals back tothe BSC directing the BSC to assign an air interface traffic channel foruse by the mobile station. For a voice call, the MSC uses well-knowncircuit protocols to signal call setup and establish a circuitconnection to a destination switch that can then connect the call to acalled device (e.g., landline phone or another mobile station). For apacket-data session, the BSC signals to the PDSN 216 by way of PCF 214.The PDSN 216 and mobile station 102 then negotiate to establish a datalink layer connection, such as a point to point protocol (PPP) session.Further, the PDSN 216 sends a foreign agent advertisement that includesa challenge value to the mobile station, and the mobile station 102responds with a mobile-IP registration request (MIP RRQ), including aresponse to the challenge, which the PDSN forwards to HA 222. The HAthen assigns an IP address for the mobile station 102 to use, and thePDSN passes that IP address via the BSC to the mobile station.

Generally, it should be understood that the depiction of just one ofeach network element in FIG. 1 is illustrative, and there could be morethan one of any of them, as well as other types of elements not shown.The particular arrangement shown in FIG. 1 should not be viewed aslimiting with respect to the present invention. Further, the networkcomponents that make up a wireless communication system such as system100 are typically implemented as a combination of one or more integratedand/or distributed platforms, each comprising one or more computerprocessors, one or more forms of computer-readable storage (e.g., disksdrives, random access memory, etc.), one or more communicationinterfaces for interconnection between elements and the network andoperable to transmit and receive the communications and messagesdescribed herein, and one or more computer software programs and relateddata (e.g., machine-language instructions and program and user data)stored in the one or more forms of computer-readable storage andexecutable by the one or more computer processors to carry out thefunctions, steps, and procedures of the various embodiments of thepresent invention described herein. Similarly, a communication devicesuch as exemplary mobile station 102 typically comprises auser-interface, I/O components, a communication interface, a tonedetector, a processing unit, and data storage, all of which may becoupled together by a system bus or other mechanism.

Throughout this description, the term “base station” may be used torefer to a BTS, a BSC, or a combination of one or more BTSs and a BSC,for instance. Further, it should be understood that actions that aregenerally described as being carried out by the RAN (or simply by the“network”) may be carried out by various different entities orcombinations of entities in the RAN. Moreover, actions described asbeing carried out by one RAN entity may also be carried out by other RANentities, in whole or in part, without departing from the scope of theinvention.

II. General Paging Functionality

Each BTS in a RAN, such as BTS 104, preferably broadcasts aspecification of paging channels defined in the BTS's respectivecoverage area. As such, a mobile station beginning to operate in a givensector can determine how many paging channels the coverage area includesand, if more than one, can determine which paging channel to monitor. Byway of example, this specification may be an indication in an overheadmessage referred to as a system parameters message, which the basestation broadcasts periodically or repeatedly. Further, thespecification may define the paging channels expressly by particularcoding parameters or, as noted above may simply indicate how many pagingchannels there are, which may inherently define the paging channels if asequence of possible paging channels exists.

A mobile station that has no active data session or voice call (i.e., noassigned traffic channel), but is otherwise operational, is said to bein an “idle” state or mode. While in the idle state, the mobile stationperiodically scans the air interface for communication from the wirelesscommunication system. More specifically, the mobile station monitors thepaging channel for page messages from its serving base station and/orother base stations. Page messages are used to send the mobile stationinformation, alerts, and requests during times when the mobile stationisn't engaged in a communication (i.e., when the mobile station isidle). For instance, the mobile station may be alerted of an incomingcall or other incoming communications via a page message.

Accordingly, a RAN may be configured to page a mobile station bytransmitting a page message over a paging channel. Paging is typicallyinitiated when an MSC receives an indication that a mobile stationshould be paged (e.g., an incoming call), or when the MSC itselfdetermines that a mobile station should be paged. The indicationnormally includes an identifier of the mobile station, such as a NetworkAccess Identifier (NAI), a Mobile Directory Number (MDN), a MobileIdentification Number (MIN), an International Mobile SubscriberIdentifier (IMSI), an electronic serial number (ESN), and a mobileequipment identifier (MEID). The MSC may then transmit somerepresentation of the indication to the appropriate BTS or BTSs. Inturn, the BTS may use the mobile-station identifier to generate a pagerecord, which the BTS may then place in a general page message (GPM) fortransmission to the mobile station.

Each page record may contain error detection bits, such as a checksum.Thus, once the mobile station receives such a GPM, the mobile stationtypically will validate the checksum. If the checksum indicates thatthere are no bit errors in the page record, the mobile station may replywith a page-response message (PRM) on the access channel. Afterreceiving the PRM, the RAN may then assign the one or more trafficchannels to the mobile station. However, if the checksum indicates thatone or more bits in the page record are in error, the mobile station maydiscard the GPM.

In an exemplary embodiment, each base station (e.g., BTS 104 and/or BSC106) may include a scheduler 126, which functions to schedule pagerecords received from switches, such as MSC 108, as they are received.As such, the scheduler 126 may function to group page records into GPMs,and schedule the transmission of the GPMs during appropriatepaging-channel slots. While the scheduler 126 is shown as beingconnected to BSC 106, it should be understood that a scheduler may alsobe connected directly to a BTS 104. It should also be understood thatwhile scheduler 126 is depicted as a separate entity from BTS 104 andBSC 106, a scheduler may be integrated as hardware, software, and orfirmware in a BSC or in a BTS. Furthermore, it is possible that ascheduler may be implemented in another configuration altogether (suchas a system-wide scheduler, for instance).

A RAN providing service under IS-2000 typically implements multiplepaging channels (in practice, up to seven are usually provided), witheach paging channel being subdivided into 2,048 periodically recurring,80-millisecond (ms) slots. Under IS-2000, a recurring “slot cycle” thatincludes a predetermined number of timeslots is defined within a2,048-slot “maximum slot cycle” period (e.g., 163.84 seconds). Herein, aslot cycle may also be referred to as a “paging-channel slot cycle.” Theslot cycle is set according to a system parameter called the slot cycleindex (SCI). For instance, slot cycle index values of 0, 1, and 2 defineslot cycles of 1.28, 2.56, and 5.12 seconds, respectively (i.e., 16, 32,and 64 slots, respectively) per maximum slot cycle. In existing IS-2000networks, an SCI of 2 is typically used, and therefore each paging slotcycle is 5.12 seconds, or 64 slots, in duration. However, it iscontemplated that exemplary embodiments may be implemented inconjunction with any maximum slot cycle period, any SCI, and possiblyentirely different paging schemes as well.

In this context, each mobile station 102 is typically assigned a certainslot within the paging slot cycle by an appropriate scheduler 126. Forinstance, when a mobile station connects to a given base station underIS-2000, the mobile station is typically assigned to a certain slot inthe slot cycle based on the mobile station's International MobileStation Identifier (IMSI). Further, the RAN may implement a “hashingfunction” that effectively randomizes the selection of slots such that,on average, no one slot in the slot cycle is assigned to substantiallymore mobile stations than any other slot. Other techniques for assigningslots to mobile stations may be used as well.

Accordingly, to help conserve battery power, a mobile station 102typically monitors only its assigned slot on a given paging channel. Inpractice, the duration of the scan is typically 80-120 ms, althoughlonger or shorter scans are possible depending on the structure of thepaging channel, whether a message has been sent, the length of themessage, and the RF conditions on the mobile station's forward link,among other factors.

Furthermore, an exemplary mobile station 102 may or may not monitor itsassigned slot in every slot cycle, depending upon the mobile station'slocation relative to where the mobile station registered. For example,when mobile station 102 is located near to where it last registered withthe RAN, the mobile station may only scan its assigned slot only inevery other slot cycle, but when further from where it last registered,scan in its assigned slot in every slot cycle. For instance, when theslot cycle is 5.12 seconds and mobile station 102 is located near towhere it last registered, the mobile station may scan the paging channelevery 10.24 seconds for any pages directed to the mobile station. Whenthe mobile station is further from where it last registered, however,the mobile station may scan the paging channel every 5.12 seconds.

III. Resending Pages and Zone-Based Paging

In a further aspect of paging, a RAN will typically resend a page (andpossibly do so a number of times) when a page fails to reach (orseemingly fails to reach) a mobile station. More specifically, when amobile station receives a page, it typically responds to the RAN with apage response message. Therefore, if the RAN does not receive the pageresponse, the RAN deduces that a failure of some sort occurred in pagingthe mobile station. The failure could result from the mobile stationbeing temporarily out of coverage or for some other reason. When thisoccurs, the switch in the serving system (e.g., MSC 108) preferablyattempts to re-page the mobile station. In particular, an MSC 108 maywait a predetermined period of time (five seconds, for instance) toreceive an indication from a base station, which indicates a pageresponse message has been received. If the MSC 108 does not receive suchan indication, the MSC will resend the page to the base station to betransmitted to the mobile station. The MSC will repeat this processuntil a page response message indicates that the page was successfullyreceived, or the maximum number of attempts has been made withoutreceiving a page response message, and the page is deemed to havefailed.

Furthermore, when a RAN seeks to page a mobile station 102 (e.g., for anincoming call or for some other reason), a switch, such as MSC 108, maysend a page to numerous base stations in the switch's coverage area,such as BTS 104, with the hope that when the base stations broadcast thepage message, the mobile station 102 will receive the page message in atleast one of the associated sectors, and will respond. Given thescarcity of paging channel resources in most modern cellular networks,paging across multiple sectors is typically implemented with amore-targeted paging process that is commonly referred to as “zone-basedpaging.”

With zone-based paging, a cellular network is divided into paging zones,each with a respective zone ID, and paging is performed on a zone-basis.To facilitate this, each base station (e.g., BTS 104) in the system maybroadcast as one of its overhead parameters the zone ID for the zone inwhich the base station is located. A mobile station 102 operating in thenetwork may then programmatically monitor the zone IDs indicated in theoverhead messages and may register with the network when they detectthat they have moved into a new zone, or for other reasons.

With this process, the registration records thereby maintained byswitches and/or home location registers will indicate the paging zone inwhich each mobile station last registered. When a switch (e.g., MSC 108)seeks to page a mobile station, the switch may then efficiently send thepage message to just those base stations that are within the zone of themobile station's last registration, as it is likely that the mobilestation is in that zone. Further, the switch may send the page messageto the base stations in zones adjacent to the mobile station's zone oflast registration, to cover the possibility that the mobile station hasmoved to a new zone but has not yet registered its presence in the newzone.

Using the general paging strategy that is implemented in many RANsproviding IS-2000 and/or EVDO service, the network makes up to threeattempts to page a mobile station. In particular, the MSC 108 mayinitiate a first attempt by sending a page record to one or more basestations for transmission in the paging zone in which the mobile stationis registered (i.e., the base stations in the paging zone in which themobile station is registered transmit the page record). Then, if thefirst attempt is unsuccessful (i.e., the mobile station does notacknowledge the page record), the MSC initiates a second attempt byagain sending the page record to the one or more base stations fortransmission in the paging zone in which the mobile station isregistered, and possibly one or more additional base stations fortransmission in one or more adjacent zones as well. If the secondattempt also fails, then the MSC initiates a third attempt to page themobile station, which typically involves sending the page record tomultiple base stations for a system-wide transmission of the page record(i.e., in all paging zones), although it is possible that a thirdattempt may be of a different scope as well.

FIG. 2 is a block diagram illustrating a portion of a coverage area ofradio access network 12, which is divided into paging zones according toa zone-based paging scheme. As shown, the base stations 14-30 aregrouped into zones A, B, and C to facilitate zone-based paging asdescribed above. In particular, each base station may broadcast a zoneID indicating its zone, and a mobile station such as mobile station 102may monitor the zone IDs broadcast in the coverage areas where themobile station is operating. When the mobile station detects a change inzone ID, the mobile station may then responsively register its presencein the new zone, so that the RAN would then know to page the mobilestation in that new zone.

In an exemplary embodiment, zones are defined by MSCs (i.e., zone IDsare created, base stations are assigned to a particular zone ID, etc.).For instance, MSC 52 may create zone IDs corresponding to each zone A-C,and assign base stations 14-16 to zone A, base stations 18-24 to zone B,and base stations 26-30 to zone C. It should be understood, however,that other network entities may define paging zones, in combination withor instead of MSC 52, without departing from the scope of the invention.

As shown, MSC 52 also includes or has access to mobile stationregistration data 66. The mobile station registration data 66 preferablycomprises data that specifies per mobile station where the mobilestation is currently registered, such as the zone in which the mobilestation is currently registered, among possibly other information. Morespecifically, the mobile station registration data may take the form ofa visitor location register (VLR) database, which holds a record permobile station in the MSC's service area. The mobile station's currentzone of registration can thus be indicated in the mobile station's VLRrecord. Alternatively or additionally, the mobile station registrationdata can take the form of a home location register (HLR) database thatis accessible via the MSC 52. Still alternatively, the data may bestored internally at the MSC and/or base stations, or elsewhere in someother form.

Generally, it should be understood that paging schemes other thanzone-based paging may be employed, without departing from the scope ofthe invention. For instance, a base station may be paged only in thesector in which it is registered, or only in the zone in which it isregistered. As such, it is also possible that a base station may thetrack the paging-attempt status, since such embodiments may not involvecoordination between multiple base stations that send a given page undera zone-based paging scheme. As an example of another alternativeembodiment, it is even possible that an exemplary embodiment may beimplemented in a system in which no re-paging is performed—in this case,each page may simply be treated as the last attempt to send a givenpage.

IV. Exemplary Methods and Systems

FIG. 3 is a flow chart illustrating a method 300 according to anexemplary embodiment. Method 300 involves a mobile station determiningthe location of the mobile station in relation to a zone of lastregistration, as shown by block 302. The mobile station then uses itslocation in relation to its zone of last registration as a basis fordetermining a scan period, as shown by block 304. According to anexemplary embodiment, the “scan period” indicates the duration of timethat the mobile station should wait between consecutive paging-channelscans. The mobile station then proceeds to periodically scan the pagingchannel according to the determined scan period, as shown by block 306.

An exemplary method 300 may be implemented by a mobile station that isoperating in a RAN in which a zone-based paging scheme is implemented.For example, the RAN may be configured to initially send a page to amobile station in the mobile station's zone of last registration. Thenif the initial attempt is unsuccessful, the RAN may make a secondattempt to send the page in both the zone of last registration and afirst expanded paging area (e.g., neighbor zones of the zone of lastregistration). And if the second attempt is unsuccessful, then the RANmay expand the third and final attempt to send the page to a secondexpanded paging area, as well as the zone of last registration and thefirst expanded paging area. In an exemplary embodiment, the thirdattempt may be a system-wide, and thus may be sent in all zones that areserved by the same switch that serves the zone of last registration.

Therefore, the mobile station's location relative to the zone of lastregistration may be determined to be either: (a) in the zone of lastregistration, (b) in the first expanded paging area, or (c) in thesecond expanded paging area. In an embodiment where the RAN implementsan exemplary zone-based paging scheme, the mobile station may consideritself to be located in the first expanded paging area when the mobilestation determines that it is located in a neighbor zone of its zone oflast registration. Further, the mobile station may consider itself to belocated in the second expanded paging area when it determines that it islocated in a zone which is served by the same switch as serves its zoneof registration and is separated from its zone of last registration byat least one zone. Zones in the second expanded paging area may bereferred to herein as “outer zones.”

In an alternative embodiment, the mobile station's location relative tothe zone of last registration may be determined to simply be eitherwithin the zone of last registration or outside the zone of lastregistration. The location in relation to the zone of last registrationmay be defined in other ways as well, without departing from the scopeof the invention.

In practice, a mobile station may be provided with information regardingthe location of a given zone relative to the mobile station's zone oflast registration by the RAN. For example, when a mobile stationregisters in a given zone, the RAN may send the mobile station azone-information message. The zone-information message may be created bythe switch that serves the zone in which the mobile registers, and thensent by the switch to the mobile station's serving BTS for transmissionto the mobile station.

Since the coverage area of a switch typically includes multiple zones,with each zone including one or more sectors, a zone-information messagemay provide an indication of the location of each sector in relation tothe zone of registration. As one specific example, the zone-informationmessage may indicate for each sector served by the switch, whether thesector is: (a) in the zone in which the mobile station just registered,(b) in the first expanded paging area for the mobile station (i.e., in aneighbor zone of the zone in which the mobile station just registered),or (c) in the second expanded paging area for the mobile station (i.e.,in an outer zone relative to the zone in which the mobile station justregistered). Provided with such a zone-information message, a mobilestation may be able to determine its current location in relation to itszone of last registration by determining the sector in which it iscurrently located, and then determining the location of the sector inrelation to its zone of last registration.

Based on its location relative to the zone of last registration, amobile station may expect that the RAN will attempt to send a page moreor less frequently. Accordingly, the mobile station may adjust its scanperiod to be longer or shorter depending upon whether it expects theRAN's attempts to resend a page to be more or less frequent,respectively. For example, under an exemplary zone-based paging scheme,if the mobile station is located in its zone of last registration, thenthe mobile station is in a location where all three paging attempts willbe sent (if a second or third attempt is necessary). Accordingly, themobile station knows that if it misses a page on the first attempt, oreven on the first and the second attempt, it is still likely that themobile station will receive the page on the third attempt. Therefore, ifthe mobile station determines that it is located in the zone of lastregistration, then the mobile station may determine the duration of thescan period to be three paging-channel slot cycles. With this scanperiod, the mobile station will scan the paging channel in the mobilestation's assigned slot in every third paging-channel slot cycle.

Furthermore, if the mobile station is located in a neighbor zone, thenthe mobile station is in a location where it does not expect that a pagewill be sent to it on the initial attempt. However, because the mobilestation is located in the neighbor zone, the mobile station expects thatthe page will be sent to it on the second attempt and/or on the thirdattempt (if a third attempt is necessary). Since the page should be sentto it in two out of three consecutive slot cycles, the mobile stationshould receive the page on at least the second attempt or third attempt,so long as the mobile station scans the paging channel in every otherpaging-channel slot cycle. Accordingly, if the mobile station determinesthat it is located in a neighbor zone, then the mobile station maydetermine the duration of the scan period to be two paging-channel slotcycles. With this scan period, the mobile station will scan the pagingchannel in the mobile station's assigned slot in every secondpaging-channel slot cycle.

Yet further, if the mobile station is located in an outer zone, which isseparated from the zone of last registration by at least one zone, thenthe mobile station is in a location where it expects that a page willnot be sent to it until the third attempt to send the page. Thus, themobile station does not expect to be sent a page on the initial attemptor on the second attempt. Therefore, if the mobile station does not scanthe paging channel in every paging-channel slot cycle, there is asignificant possibility that the mobile station will miss a pagealtogether. Accordingly, if the mobile station determines that it islocated in the second expanded paging area, then the mobile station maydetermine the duration of the scan period to be one paging-channel slotcycle.

The foregoing embodiment in which the mobile station adjusts its scanperiod between one and three slot cycles is most appropriate in a systemwhere the RAN's attempts to send a page occur in consecutive slotcycles. However, due to the timing with which a switch initiatessubsequent attempts to send a page, attempts may or may not be made inconsecutive slot cycles. In particular, a base station's scheduling of asubsequent attempt to send a given page depends upon when the switchresends the page to the base station, and thus depends upon the periodof time a switch will wait for a page response message before initiatingthe subsequent attempt. For example, in some IS-2000 networks, MSCs maybe configured to wait five seconds between attempts. As such, by thetime the MSC waits five seconds and then indicates to a base station tomake a subsequent attempt to send the page, the mobile station'sassigned slot in the next slot cycle (i.e., 5.12 seconds after the firstattempt) may have passed, and as such, the next attempt may be scheduledfor the mobile station's assigned slot, two slot cycles later (i.e.,10.24 seconds after the initial attempt).

When paging attempts are separated by two slot cycles, a mobile stationmay vary its scanning process accordingly. For instance, the mobilestation may change the way in which it varies the scan period, and mayscan in two consecutive slot cycles after each scan period. As aspecific example, when the mobile station is located in its zone of lastregistration, the mobile station may set the scan period to be four slotcycles. Accordingly, the mobile may scan during its assigned slot in twoconsecutive slot cycles, and then wait for four slot cycles before againscanning in two consecutive slot cycles, and so on. Further, when themobile station is located in the first expanded paging area, the mobilestation may set the scan period to be two slot cycles, and thusalternate between scanning its assigned slot in two consecutive slotcycles and refraining from scanning in the next two slot cycles. Itshould be understood that a mobile station may vary its scan period andscanning process in other ways to account for other variations in themanner in which a RAN resends pages, without departing from the scope ofthe invention.

In a further aspect, an exemplary method may be initiated when a mobilestation decides to switch to a power-save mode in order to conservebattery power. In particular, when the mobile station decides to switchto the power-save mode, the mobile station may determine its scan periodas shown in block 304. Then, while operating in the power-save mode, themobile station will scan according to the determined scan period, asshown in block 306. Thus, in order to save power, the mobile stationpreferably increases the scan period as much as possible. For example,the mobile station may determine if it can increase its scan period suchthat it will only scan in its assigned slot in every other or everythird slot cycle, depending upon the mobile station's location relativeto its zone of last registration.

A mobile station may determine that it should switch from a standardoperating mode to a power-save mode for any number of reasons. Forexample, a mobile station may switch to the power-save mode and adjustits scan period accordingly whenever the mobile station detects that itsremaining battery power has fallen below a threshold power level. FIG. 4is a flow chart illustrating an exemplary method 400 that may beimplemented by a mobile station to adjust its scanning process in aneffort to conserve its battery power.

More specifically, method 400 involves a mobile station determining itsbattery-power level, as shown by block 402. The mobile station thendetermines whether or not its battery-power level is below a thresholdlevel, as shown by block 404. If the mobile station's battery-powerlevel is above the threshold level, then the mobile station selects afirst scanning process, as shown by block 406. The first scanningprocess, which is selected when the mobile station's battery-power levelis above the threshold level, is preferably the standard process wherethe mobile station scans in its assigned slot in every paging-channelslot cycle. If, on the other hand, the mobile station's battery-powerlevel is below the threshold level, then the mobile station selects asecond scanning process, as shown by block 408. As shown, the secondscanning process preferably involves the mobile station varying its scanperiod based on the location of the mobile station in relation to thezone of last registration. (Note that it is a matter of engineeringdesign choice as to whether the first or second scanning process isselected when the battery-power level is equal to the threshold level.)In either case, the mobile station then scans the paging channelaccording to the selected scanning process, as shown by block 410.

When the second scanning process is selected, the mobile stationpreferably reduces how often it scans when possible. For instance, amobile station may scan less often by increasing its scan period. Thismay reduce the overall number of scans performed by the mobile station,which may in turn reduce the cumulative amount of power consumed toperform the scans. In an exemplary embodiment, whether or not the mobilestation can increase its scan period, and if so by how much, ispreferably a function of the location of the mobile station in relationto its zone of last registration. In particular, the scan period may bedetermined based upon the location of the mobile station in relation toits zone of last registration, as described in reference to FIG. 2herein.

It should be understood that, in addition to an adjusted scan period,operation in the power-save mode may also involve a mobile stationreducing or possibly even eliminating various other mobile-stationfunctions that tend to consume battery power. As one example, a mobilestation may register with a RAN less frequently when operating in thepower-save mode. Other examples are also possible.

FIG. 5A is a flow chart illustrating an exemplary method 500 forregistration and scanning while in a power-save mode. As shown by block502, method 500 involves the mobile station detecting that it has movedto a new zone. The mobile station then determines whether the new zoneis its zone of registration, a zone in the first expanded coverage area,an outer zone in the second expanded coverage area, or an out-of-system(OOS) zone, as shown by block 504.

If the mobile station detects that it is in its zone of lastregistration, then the mobile station only scans its assigned slot inevery third slot cycle, as shown by block 506, and refrains fromre-registering with the RAN, as shown by block 508. If the mobilestation detects that it has traveled into a zone in the first expandedpaging area, then the mobile station scans every second slot cycle, asshown by block 510. However, in an effort to conserve battery power byreducing the overall number of times a mobile station needs to register,the mobile station does not re-register in the first expanded pagingarea, as again shown by block 508. Further, while the mobile station mayscan in every slot cycle when it detects that it has traveled into azone in the second expanded paging area, as shown by block 512, themobile station may still attempt to conserve battery power by refrainingfrom re-registering with the RAN, as again shown by block 508.

Therefore, when a mobile station implements method 500, the mobilestation will only re-register with the RAN when it moves from oneserving system to another, and thus has entered an “out-of-system” zone.Herein, an out-of-system (OOS) zone may be any zone that is served by adifferent serving system from the serving system that serves a mobilestation's zone of last registration. As shown, when the mobile stationdetects that it has traveled into an out-of-system zone, the mobilestation re-registers with the RAN, as shown by block 514. Since themobile station has re-registered in the out-of-system zone, this zonenow becomes the mobile station's zone of last registration (and thusceases to be considered an out-of-system zone for the mobile station).Accordingly, the mobile station will scan only in every third slotcycle, as shown by block 516.

FIG. 5B is a flow chart illustrating another exemplary method 550 forregistration and scanning while in a power-save mode. A mobile stationthat implements method 550 may re-register more frequently than it wouldif it were using method 500. However, re-registering more frequently mayresult in a mobile station being located in its zone of lastregistration more frequently, which means a mobile station may belocated in its zone of last registration more often. Since a mobilestation may scan less frequently when located in its zone of lastregistration, the reduction in power consumption from less-frequentscanning may offset the increase in power consumption from more-frequentre-registration. Furthermore, depending upon the movement of a givenmobile station, the reduction in power consumption resulting fromless-frequent scanning may partially or wholly offset, or even exceed,the increase from more-frequent re-registration. Therefore, it ispossible that method 550 may or may not result in a greater overallreduction in power-consumption as compared to method 500, depending uponthe manner in which a given mobile station moves about.

Turning to the specifics of method 550, the method involves a mobilestation detecting that it has moved to a new zone, as shown by block552. The mobile station then determines whether the new zone is its zoneof registration, a zone in the first expanded coverage area, a zone inthe second expanded coverage area, or an out-of-system (OOS) zone, asshown by block 554. The actions taken when a mobile station detects thatit is in its zone of last registration, and when a mobile stationdetects that it is in the first expanded paging area of its servingsystem, are the same in method 550 as in method 500, as shown by blocks556-560. Furthermore, actions taken when a mobile station detects thatit is in an OOS zone are also the same in method 550 as in method 500,as shown by block 564-566.

However, according to method 550, when the mobile station detects thatit has traveled into a zone in the second expanded paging area, themobile station re-registers with the RAN, as again shown by block 564.Since the mobile station has re-registered in the zone in the secondexpanded paging area, this zone now becomes the mobile station's zone oflast registration (and thus ceases to be considered part of the mobilestation's second expanded paging area). Accordingly, the mobile stationwill scan only in every third slot cycle, as again shown by block 566.Therefore, even though the mobile station uses some additional power tore-register (as compared to method 500, where the mobile stationrefrains from re-registering in the second expanded paging area), there-registration also allows the mobile station to reduce its powerconsumption by scanning the paging channel less frequently.

FIG. 6 is a block diagram illustrating movement of an exemplary mobilestation 602 through a coverage area of a RAN. As shown, the mobilestation 602 is operating in a coverage area that includes a firstserving system 604 and a second serving system 606. The first servingsystem 604 is configured to provide service in zones A-K, and the secondserving system 606 is configured to provide service in zones Q-S.Herein, a “serving system” includes a switch and the other RAN entitiesthat operate together with the switch to provide service in the coveragearea of the switch. For example, serving system 604 includes an MSC 605that is configured to provide service in a coverage area that includeszones A-K. Accordingly, serving system 604 may also include one or moreBTSs (not shown) that each provide service in one or more sectors in thelarger coverage area of MSC 605, and one or more BSCs (not shown) thatcontrol the BTSs in the serving system. Serving system 606 likewiseincludes an MSC 607, and may also include one or more BTSs and/or one ormore BSCs in order to provide service in the coverage area that includeszones A-K.

In the illustrated scenario, mobile station 602 travels throughout thecoverage area of the RAN along path 608. More specifically, the mobilestation 602 is initially located in zone A, which is initially its zoneof last registration, and then travels along path 608. Moving along path608, mobile station 602 moves from zone A to zone D, then from zone D tozone E, then from zone E back to zone A, then from zone A to zone F,then from zone F to zone H, then from zone H to zone I, and finally fromzone I to zone Q.

To illustrate an exemplary method in practice, consider a scenario wheremobile station 602, while located in zone A, determines that its batterypower is low and responsively switches to a power-save mode according tomethod 500 of FIG. 5A. In this scenario, since zone A is the mobilestation's zone of last registration, mobile station 602 only scans thepaging channel in every third slot cycle. When mobile station 602 movesfrom zone A to zone D, which is one of the mobile station's neighborzones (i.e., a zone in the mobile station's first expanded paging area),the mobile station scans the paging channel in every second slot cycle.Further, because zone D is in the mobile station's first expanded pagingarea, the mobile station refrains from re-registering in zone D.Further, when mobile station 602 moves from zone D to zone E, which isanother one of the mobile station's neighbor zones, mobile station 602continues to scan in every second slot cycle and refrains fromre-registering in zone E. Mobile station 602 then moves from zone E backto zone A, which is its zone of registration. Accordingly, mobilestation reduces the frequency of its scans to once every three slotcycles, and continues to refrain from re-registering. After mobilestation 602 moves along path 608 and back into zone A, mobile station602 then moves into zone F, which is yet another one of the mobilestation's neighbor zones. As such, mobile station will again switch toscanning every other slot cycle, and will continue to refrain fromre-registering.

Moving further along path 608, mobile station 602 travels out of zone Fand into zone H, which is in the mobile station's second expanded pagingarea (i.e., a zone that is served by the same serving system as servesthe mobile station's zone of last registration, but is separated fromthe zone of last registration by at least one zone). Since zone H is inthe mobile station's second expanded paging area, mobile station 602will scan in its assigned slot in every slot cycle, so long as themobile station remains in zone H. Further, according to method 500,mobile station 602 refrains from re-registering in zone H. Likewise,when mobile station 602 moves from zone H to zone I, which is also inthe mobile station's second expanded paging area, the mobile stationcontinues scanning the paging channel in every one of its assignedslots, and again refrains from re-registering.

Note that when mobile station 602 implements method 500, the mobilestation will not re-register until it enters a zone that is not servedby the first serving system 604. As a result, zone A remains the mobilestation's zone of last registration while the mobile station 602 movesout of zone A, through zones D and E and back to zone A, then back outof zone A and through zones F and H into zone I. However, as the mobilestation continues along path 608, the mobile station travels out of zoneI and into zone Q, which is served by the second serving system 606.Therefore, mobile station 602 detects that it has traveled into anout-of-system zone. As a result, the mobile station will re-register inzone Q, which effectively makes zone Q its new zone of lastregistration. Thus, once mobile station 602 registers in zone Q, mobilestation 602 may begin to scan the paging channel only once in everythree slot cycles.

Further, when mobile station 602 registers in zone Q with the secondserving system 606, the second serving system 606 may send updated zoneinformation to mobile station 602. In particular, a zone-informationmessage may identify sectors in zone Q as being in the zone of lastregistration, sectors in zone R as being in the first expanding pagingarea, and sectors in zone S as being in the second expanded paging area.The updated zone information may not include any information regardingzones A-K since these zones are not served by the second serving system606. Therefore, the mobile station may assume from that the lack of anyindication for any of zones A-K, that these zones are all OOS zones.

To illustrate another exemplary method in practice, consider a differentscenario where mobile station 602, while located in zone A, determinesthat its battery power is low and responsively switches to a power-savemode according to method 550 of FIG. 5B. According to method 550, themobile station operates in the same manner as it would under method 500,except that the mobile station will re-register when it enters a zone inits second expanded paging area.

As mobile station 602 moves along path 608 out of zone A, through zonesD and E and back to zone A, and then back out of zone A into zone F,mobile station 602 is either in its zone of last registration (zone A)or a neighbor zone (zone D, E, or F). Therefore, as mobile station 602moves along this portion of path 608, it refrains from re-registeringand adjusts its scan period in the same manner as it would under method500. However, when mobile station 602 travels out of zone F it movesalong path 608 into zone H, which is in the mobile station's secondexpanded paging area. And since mobile station 602 is implementingmethod 550, mobile station 602 will re-register in zone H.

When mobile station 602 re-registers in zone H, this makes zone H themobile station's zone of last registration. Thus, once mobile station602 registers in zone H, mobile station 602 may begin to scan the pagingchannel only once in every three slot cycles. Furthermore, when mobilestation 602 registers in zone H, the first serving system 604 may sendupdated zone information to mobile station 602. In particular, theupdated zone-information message may identify sectors in zone H as beingin the zone of last registration, sectors in zones E, F, and I as beingin the first expanded paging area, and sectors in zones A-D, G-H, andJ-K as being in the second expanded paging area. Accordingly, whenmobile station 602 moves along path 608 out of zone H and into zone I,zone I will be a neighbor sector. Accordingly, mobile station 602 willscan every other slot cycle and refrain from re-registering in zone I.

It should be understood the arrangements and functions described hereinare presented for purposes of example only, and that numerous variationsare possible. For instance, elements can be added, omitted, combined,distributed, reordered, or otherwise modified. Further, where thisdocument mentions functions that can be carried out by a device or otherentity, it should be understood that the functions may be implemented bysoftware (e.g., machine language instructions stored in data storage andexecutable by a processor), firmware, and/or hardware.

We claim:
 1. A method comprising: (a) at a mobile station that isoperating in a radio access network, determining a location of themobile station in relation to a zone of last registration, wherein themobile station last registered with the radio access network in the zoneof last registration, and wherein the zone of last registration is apaging zone in the radio access network; (b) the mobile station usingthe location of the mobile station in relation to the zone of lastregistration as a basis for determining a scan period to wait betweenscans of a paging channel for a page from the radio access network; and(c) the mobile station periodically scanning the paging channelaccording to the determined scan period.
 2. The method of claim 1,further comprising: the mobile station initially making a determinationto switch to operating in a power-save mode; and in response to thedetermination to switch to operating in the power-save mode, the mobilestation carrying out (a)-(b), switching to the power-save mode, andcarrying out (c) while in the power-save mode.
 3. The method of claim 2,wherein making the determination to switch to operating in thepower-save mode comprises making a determination that a power level ofthe mobile station is below a threshold power level.
 4. The method ofclaim 1: wherein the radio access network is configured to initiallypage the mobile station in the zone of last registration, and then ifunsuccessful, to page the mobile station in both the zone of lastregistration and a first expanded paging area; and wherein determiningthe location of the mobile station in relation to the zone of lastregistration comprises determining whether the mobile station is locatedin either (a) the zone of last registration or (b) the first expandedpaging area.
 5. The method of claim 1: wherein the radio access networkis configured to: (a) initially page the mobile station in the zone oflast registration, (b) then if unsuccessful, page the mobile station inboth the zone of last registration and a first expanded paging area, and(c) then if again unsuccessful, page the mobile station in the zone oflast registration, the first expanded paging area, and a second expandedpaging area; and wherein determining the location of the mobile stationin relation to the zone of last registration comprises determiningwhether the mobile station is located in either: (a) the zone of lastregistration, (b) the first expanded paging area, or (c) the secondexpanded paging area.
 6. The method of claim 5, wherein using thelocation of the mobile station in relation to the zone of lastregistration as a basis for determining the scan period comprises: ifthe mobile station is located in the zone of last registration, thendetermining the scan period to be a duration of three paging-channelslot cycles; if the mobile station is located in the first expandedpaging area, then determining the scan period to be a duration of twopaging-channel slot cycles; and if the mobile station is located in thesecond expanded paging area, then determining the scan period to be aduration of one paging-channel slot cycle.
 7. The method of claim 6,wherein the mobile station is assigned a slot in each paging-channelslot cycle in which the mobile station can scan the paging channel for apage from the radio access network, and wherein scanning the pagingchannel according to the determined scan period comprises: if the mobilestation is located in the zone of last registration, then scanning thepaging channel in the mobile station's assigned slot in every thirdpaging-channel slot cycle; if the mobile station is located in the firstexpanded paging area, then scanning the paging channel in the mobilestation's assigned slot in every second paging-channel slot cycle; andif the mobile station is located in the second expanded paging area,then scanning the paging channel in the mobile station's assigned slotin every paging-channel slot cycle.
 8. The method of claim 5: whereinthe first expanded paging area comprises one or more neighbor zones,wherein each neighbor zone is adjacent to the zone of last registration;and wherein the second expanded paging area comprises all zones servedby a switching system of the radio access network outside of the zone oflast registration and the first expanded paging area.
 9. The method ofclaim 5, further comprising the mobile station: detecting that themobile station has traveled into a zone in the first expanded pagingarea; and while the mobile station is located in the zone first expandedpaging area, the mobile station refraining from re-registering with theradio access network.
 10. The method of claim 9, further comprising themobile station: detecting that the mobile station has traveled into azone the second expanded paging area; and while the mobile station islocated in the zone in the second expanded paging area, the mobilestation continuing to refrain from re-registering with the radio accessnetwork.
 11. The method of claim 9, further comprising the mobilestation: detecting that the mobile station has traveled into a zone inthe second expanded paging area; and in response to determining that themobile station is located in the zone in the second expanded pagingarea, the mobile station re-registering in the zone in the secondexpanded paging area.
 12. The method of claim 1, further comprising themobile station initially receiving a zone-information message from aradio access network, wherein a given switch provides service in acoverage area that comprises a plurality of zones that includes the zoneof last registration, wherein each of the plurality of zones comprisesone or more sectors, and wherein the zone-information message comprisesan indication of the location of each of the sectors in relation to thezone of registration; and wherein determining the location of the mobilestation in relation to the zone of last registration comprises:determining a sector in which the mobile station is currently located;and determining the location of the mobile station in relation to a zoneof last registration to be the location of the sector in which themobile station is currently located in relation to a zone of lastregistration.
 13. A method comprising: a mobile station determining abattery-power level of the mobile station, wherein the mobile stationhas most-recently registered with a radio access network in a zone oflast registration, and wherein the zone of last registration is a pagingzone in the radio access network; the mobile station determining whetherthe determined battery-power level is below a threshold level; if thedetermined battery-power level is above the threshold level, then themobile station selecting a first scanning process; if the determinedbattery-power level is below the threshold level, then the mobilestation selecting a second scanning process, wherein the second scanningprocess varies a period between scans of a paging channel based at leastin part on a location of the mobile station in relation to the zone oflast registration; and scanning the paging channel according to theselected scanning process.
 14. The method of claim 13, wherein themobile station is assigned a slot in each paging-channel slot cycle inwhich the mobile station can scan the paging channel for a page from theradio access network, wherein the radio access network is configured to:(a) initially page the mobile station in the zone of last registration,(b) then if unsuccessful, page the mobile station in a first expandedpaging area, and (c) then if again unsuccessful, page the mobile stationin a second expanded paging area; and wherein the second scanningprocess comprises: the mobile station determining whether the mobilestation is located in either: (a) the zone of last registration, (b) thefirst expanded paging area, or (c) the second expanded paging area; andif the mobile station is located in the zone of last registration, thenthe mobile station scanning the paging channel in the mobile station'sassigned slot in every third paging-channel slot cycle; if the mobilestation is located in the first expanded paging area, then the mobilestation scanning the paging channel in the mobile station's assignedslot in every second paging-channel slot cycle; and if the mobilestation is located in the second expanded paging area, then the mobilestation scanning the paging channel in the mobile station's assignedslot in every paging-channel slot cycle.
 15. A mobile-station systemcomprising: a non-transitory tangible computer-readable medium; andprogram instructions stored on the non-transitory tangiblecomputer-readable medium and executable by at least one processor to:(a) determine a location of the mobile station in relation to a zone oflast registration, wherein the mobile station last registered with theradio access network in the zone of last registration, and wherein thezone of last registration is a paging zone in the radio access network;(b) use the location of the mobile station in relation to the zone oflast registration as a basis to determine a scan period to wait betweenscans of a paging channel for a page from the radio access network; and(c) cause the mobile station to periodically scan the paging channelaccording to the determined scan period.
 16. The mobile-station systemof claim 15, further comprising program instructions stored on thenon-transitory tangible computer-readable medium and executable by atleast one processor to: make a determination that a mobile stationshould switch to operation in a power-save mode; and in response to thedetermination that the mobile station should switch to operation in thepower-save mode: cause the mobile station carry out (a)-(b); and causethe mobile station to carry out (c) while the mobile station is in thepower-save mode.
 17. The mobile-station system of claim 15: wherein theradio access network is configured to: (a) initially page the mobilestation in the zone of last registration, (b) then if unsuccessful, pagethe mobile station in both the zone of last registration and a firstexpanded paging area, and (c) then if again unsuccessful, page themobile station in the zone of last registration, the first expandedpaging area, and a second expanded paging area; and wherein the programinstructions stored on the non-transitory tangible computer-readablemedium and executable by at least one processor to determine thelocation of the mobile station in relation to the zone of lastregistration comprise program instructions stored on the non-transitorytangible computer-readable medium and executable by at least oneprocessor to determine whether the mobile station is located in either:(a) the zone of last registration, (b) the first expanded paging area,or (c) the second expanded paging area; and wherein the programinstructions stored on the non-transitory tangible computer-readablemedium and executable by at least one processor to use the location ofthe mobile station in relation to the zone of last registration as abasis to determine the scan period comprise program instructions storedon the non-transitory tangible computer-readable medium and executableby at least one processor to: if the mobile station is located in thezone of last registration, then determine the scan period to be aduration of three paging-channel slot cycles; if the mobile station islocated in the first expanded paging area, then determine the scanperiod to be a duration of two paging-channel slot cycles; and if themobile station is located in the second expanded paging area, thendetermine the scan period to be a duration of one paging-channel slotcycle.
 18. The mobile-station system of claim 17, wherein the mobilestation is assigned a slot in each paging-channel slot cycle in whichthe mobile station can scan the paging channel for a page from the radioaccess network, and wherein the program instructions stored on thenon-transitory tangible computer-readable medium and executable by atleast one processor to scan the paging channel according to thedetermined scan period comprise program instructions stored on thenon-transitory tangible computer-readable medium and executable by atleast one processor to: if the mobile station is located in the zone oflast registration, then scan the paging channel in the mobile station'sassigned slot in every third paging-channel slot cycle; if the mobilestation is located in the first expanded paging area, then scan thepaging channel in the mobile station's assigned slot in every secondpaging-channel slot cycle; and if the mobile station is located in thesecond expanded paging area, then scan the paging channel in the mobilestation's assigned slot in every paging-channel slot cycle.
 19. Themobile-station system of claim 17, further comprising programinstructions stored on the non-transitory tangible computer-readablemedium and executable by at least one processor to: detect when themobile station travels into the first expanded paging area; andresponsively cause the mobile station to refrain from re-registeringwith the radio access network while the mobile station is located in thefirst expanded paging area.
 20. The mobile-station system of claim 19,further comprising program instructions stored on the non-transitorytangible computer-readable medium and executable by at least oneprocessor to, while the mobile station is operating in the power-savemode: detect when the mobile station has travels into a zone in thesecond expanded paging area; and responsively cause the mobile stationto re-register with the radio access network, wherein the mobile stationre-registers in the zone in the second expanded paging area.